The induction of an immune response in a living animal causes many things to occur within that animal, one of which is the production of antibodies. There are several characteristics of an antibody response including specificity, amount, isotype (class) and affinity of the antibodies produced. The specificity determines the ability of the antibody to distinguish one immunogen from another. The amount of antibody is a function of the number of responding B cells, their rate of antibody synthesis, and the persistence of the antibody after production. The persistence of the antibody in different tissues of the body is determined by its isotype, wherein each isotype has a different half-life in vivo. The isotypic composition of an antibody response also determines the biological functions these antibodies can perform and the sites in which they will be found. Finally, the strength of binding or affinity of the antibody to its antigen is important because the higher the affinity, the less the amount of antibody that will be necessary to eliminate the antigen.
These different characteristics of the antibodies occur at different times during the development of a mature antibody. This process is known as affinity maturation. The affinity maturation process varies not only in response to each individual immunogen, but also for every individual animal which is undergoing the immune response.
In recent years, it has been determined that avians and other animal species produce different immunoglobulins (Igs) than do mammals. In particular, mammals produce a unique series of Igs, including IgM, IgG, IgE and IgA among others. In the vertebrate classes of Amphibia, Reptilia and Aves, the primary Ig which is produced in the egg yolk is IgY. IgY is believed to be the evolutionary ancestor of IgG and IgE. Not much more, however, is known about IgY, especially with regard to its functional requirements and limitations (Warr et al., Immunology Today, Vol. 16, No. 8, pp 392-298, 1995).
In the art, there is a process known as passive immunization which relates to the transfer of immunity from one species to another. For example, in a common form of passive immunization, an avian is immunized with one or more immunogens. Immunization with such immunogens induces an immune response in the avian. Antibodies to the immunogens are produced by the avian during the immune response. A large number of these antibodies become concentrated in the egg of the avian. Administration to an animal of the egg containing such antibodies, or administration of the purified antibodies themselves, causes that animal to become passively immunized to the corresponding immunogens due to the transfer of the antibodies. Such a passive immunization process is the subject of U.S. Pat. No. 4,748,018, assigned to DCV, Inc. and incorporated herein by reference.
The process of passive immunization using antibodies produced in the egg of an egg-producing animal requires, of course, the collection of antibody-containing eggs from an immunized avian. In this process, it is known in the art that the highest titers of antibodies are found in eggs produced at the early stages of the egg-laying cycle of an immunized avian. As time goes on after an immune response induction, antibody titers rise quickly, eventually peak and then fall off. The production of antibody titer in the eggs of an immunized avian has been charted to generally form a bell-shaped curve over the entire egg laying-life cycle of the avian (see R. Schade et al., ATLA, 19, pp. 403-419, 1991). Antibody titers can be somewhat maintained by revaccination. Either way, it is generally the practice in the art to collect eggs soon after vaccination or revaccination (i.e. within the first few (1-5) weeks) in order to obtain eggs having the highest antibody titers.